The invention relates generally to a method and apparatus for evaluating the fluids produced from a formation interval in a well and more particularly to such method and apparatus to be used when the fluids are pumped from the formation interval by a downhole pump.
Production logging tools for evaluating the fluid produced from a formation interval are well known. These logging tools are designed to measure several characteristics of the fluids such as the velocity, the density, the temperature and the pressure of the fluids. Standard logging tools are designed to be lowered into the well through a tubing to derive measurements from a production zone below the tubing. In order to evaluate a production interval, several passages of a combined production logging tool are carried out along the formation interval as described for example in U.S. Pat. No. 4,267,726 (Noik).
Most of the time, wells can be produced by replacing the drilling mud by a lighter fluid such as oil. The downhole well pressure is thus reduced and fluids start flowing naturally from the producing formation interval through the tubing due to the existing formation pressure. The fluids produced can be evaluated by a production logging tool lowered to the formation interval through the bottom end of the tubing which has an opening large enough for the logging tools to be moved therethrough.
However, in some circumstances, the fluids have to be pumped from the formation interval by a pump connected at the lower end of the tubing string. Known electrical submersible pumps can be used which are fed with electrical current through a cable extending along the tubing to the surface. Since the pump is at the bottom end of the tubing, the logging tools have to be lowered into the well in the annulus outside the tubing. Usual electrical pumps have a relatively important outside diameter of around 5.5 inches (14 cm). When such a pump is used in a 7 inch casing with an inside diameter of about 6.3 inches (16 cm), the clearance between the pump and the casing is less than one inch (2.5 cm) and a standard production logging tool which has a usual outside diameter of 1 11/16 inch (4.3 cm) cannot be lowered below the pump through this space. Then, a special small diameter logging tool has to be designed.
One object of the invention is to provide a method and an apparatus which allow production logging measurements to be carried out by standard logging tools in a formation interval located below a downhole pump when fluids are pumped out of the formation interval.
Another object of the invention provides a method and apparatus for evaluating the fluids produced in a well by pumping wherein a production logging tool can be run below a downhole pump in relatively small diameter wells.